To investigate how transcription termination and RNA processing may regulate the expression of complex cellular transcription units from which mRNAs with alternative 3' ends are derived, we have utilized adenovirus as an expression vector. We cloned the mouse immunoglobulin heavy chain mu constant region gene into an adenovirus-5 (Ad-5) vector. We showed for the first time that adenoviruses can productively infect human lymphoid cell lines isolated from various stages of development with reproducible kinetics. We demonstrated that the immunoglobulin gene in the vector was expressed in a cell type specific manner following infection of human B and T lymphocytic cell lines by the recombinant, mu-Ad. Furthermore, we showed also for the first time that the cell type specific expression of mu was not governed by transcription termination. Rather, it was regulated post-transcriptionally resulting in differential synthesis of the two mu specific mRNAs: the secreted-specific E1B-mu-s mRNAs and the membrane-bound specific E1B-mu-m mRNAs. Using this vector system we propose: 1. To define the pathway and kinetics of mu mRNA synthesis in vivo in lymphoid and non-lymphoid cells infected by mu-Ad, to determine if differential polyadenylation, transcript stability and transport play a role in the steady state levels of E1B-mu-s and E1b-mu-m mRNAs. 2. To identify cis-acting immunoglobulin regulatory elements. 3. To investigate trans-acting regulatory factors in vivo by determining the expression of mu-Ad and its modifications in the context of different lymphoid cells. 4. To study the mu gene in vitro by isolating transcription complexes of mu-Ad and its modifications for transcription and RNA processing in the presence of homologous and heterologous cellular extracts.